Chapter 7 - EAP

Question 1

What is the basic architecture of EAP?

Answer 1

Extensible Authentication Protocol (EAP):
- Supplicant
- Authenticator
- EAP server

Question 2

How is an EAP message structured?

Answer 2

• 8 bit code - request, response, success, failure
• 8 bit identifier
• 16 bit length
• n bit data

Question 3

How does EAP-TLS work?

Answer 3

• Uses TLS 1.3

Handshake:
- Server initiates EAP-TLS
- Supplicant sends “Client Hello” with supported algorithms, Client-DH and Client Rand
- Server computes Master Secret
- Server sends “Server Hello” with selected algorithms, server-DH, server rand + encrypted and integrity protected: Certificate Request, Server Certificate, Certificate verify, Finished
- Client checks Certificate Verify with Server Certificate
- Client computes Master Secret
- Client sends encrypted and integrity protected: Client certificate, Client verify, Finished
- Server checks Client verify with Client certificate

Question 4

What is EAP-TTLS? How can it be attacked?

Answer 4

• EAP-TLS with server authentication only
• Client is authenticated after handshake and is protected with generated keys (e.g. password based (PAP))

On EAP-TTLS with Pap:
Evil Twin with fake certificate:
- If MD doesnt check certificate or attacker has an certificate from the same root certificate
- Attacker obtains username/ password

Question 5

How does EAP-TTLS with MSChapv2 work?

Answer 5

• Client and server compute challenge in TLS handshake
• Client computes response with password and sends response and peer challenge to TTLS Server
• TTLS verifies response and forwards peer challenge to AAA-server
• AAA-server replies with peer-response using stored password

Question 6

Why is a EMSK needed in EAP?

Answer 6

EAP re-authentication:
- Large Network with roaming (e.g. eduroam) have multiple networks with local EAP server and a home EAP server.
- Local EAP server acts as a proxy
- To avoid re-authentication with home EAP server EMSK is saved on local EAP server
- Used to derive future MSKs

Question 7

How is EAP re-authentication done?

Answer 7

• Home EAP server provides local EAP server with Domain-Specific Root Key (DSRK) based of Extenden MSK (EMSK)
• Local EAP server derives Domain-Specific re-authentication Root Key (DS-rRK). It then can derive rMSK for re-authentication
• Supplicant initiates EAP re-authentication with re-authentication Integrity Key
• Local server derives rMSK and sends in EAP finish